Influence of power ultrasound on leather processing. Part I: Dyeing

Application of power ultrasound (18kHz - 1MHz) to leather processing has be come more viable due to (1) increased commercial availability and advances of ultrasound technology, (2) the apparent applicability of ultrasound acro ss the whole breadth of "wet" processes in leather manufacturing, and (3) t ightened environmental legislation which demands new efficient processing t echnology. Thus power ultrasound (38 kHz, 1.36 W cm(-2)) has been employed in our systematic investigations on a number of leather processes. In this first part of a series of papers, the effects of power ultrasound on leathe r dyeing and the factors influencing the effectiveness of ultrasonic dyeing , such as timing, mechanical agitation, temperature, type of dyes, type of leather and dye concentration, have been investigated. It was found that ul trasonic dyeing can increase the degree of dye penetration by 50-120%, shor ten the process time by 40-70%, facilitate low temperature dyeing or allow a reduction of the dye offer by 30%. In addition, it can also improve the c olor fastness particularly to wet rubbing. This remarkable enhancing effect is attributed mainly to an increased diffusion coefficient of dyestuff in leather in the presence of power ultrasound. The results have shown that ul trasound is best applied in the initial stage of the dyeing for a short per iod of time and is most effective for a dyeing system which has difficulty in achieving high dye exhaustion or penetration. It was found that ultrasou nd is more effective than mechanical stirring and can be combined with mech anical agitation in accelerating the dyeing process. The influence of tempe rature is system dependent and there exists an optimal temperature for maxi mizing the effectiveness of ultrasonic dyeing.